1. Field of the Invention
The present invention relates to a method and system for reliably, accurately and rapidly executing an automatic focusing function for a scanning charged particle microscope typified by an electron microscope, etc.
2. Description of Related Art
Tests are implemented using defect checking devices to detect defects such as the adhesion of foreign matter that are the main cause of defective device operation as a means of managing yield in semiconductor device manufacturing processes. The defect checking devices detect defects and store the number and position of these defects in a defect file for subsequent processes. Semiconductor device wafers are typically substantially circular plates as shown in FIG. 4. A lattice of a large number of the same chips 2 are transferred onto a single wafer 1. The checking device then scans the surface of the wafer manufactured in this manner using an optical probe so as to detect surface defects. When a defect is detected, a chip number (for example, a way of showing which row of which column) specifying at which chip the defect exists and internal chip coordinate information specifying the position within the chip are stored in memory as a data file. Monitoring and analysis of the defects is carried out by various microscopes and analysis apparatus based on this storage information and one of these is high resolution defect monitoring using an electron microscope for defect monitoring. During this time, positioning of the noted defects in the field of vision of the microscope is carried out based on storage information of the defect checking device. However, focal adjustment of the optical system of the microscope in order to observe the defects using an electron microscope are carried out by the electron microscope itself which is typically provided with an Auto Focus function.
Conventionally, a so-called “frame focusing control method” is widely adopted for these automatic focusing mechanisms. This frame focusing control method is a method whereby frame images are sequentially read in while moving the focal point and is based on the theory that a difference signal with neighboring pixels occurring at an outline portion is bigger for clearer images. A differential value is obtained for the image signal and the focal point is moved in a direction giving a larger value. An image expressing the maximum value is then traced or extrapolated and the in-focus point is obtained. However, a time on the order of three to ten seconds, depending on the application, is required in order to carry out the operation of taking in a large number of frame images while moving the focal point.
“Focal point adjustment methods occurring in charged particle beam device” was therefore proposed in Japanese Patent Laid-open Publication No. Hei. 7-16132 as a means for overcoming the fact that this control operation is too time-consuming. This reference discloses a focal point adjustment method for a charged particle beam device comprising a focusing lens for focusing a charged particle beam onto a sample, scanning means for scanning an irradiation position of the charged particle beam on the sample, a detector for detecting a signal obtained by irradiation of the sample with a charged particle beam, and means for sequentially changing the focusing of the charged particle beam on the sample. Here, the focusing of the charged particle beam is sequentially changed in synchronization with a vertical scanning signal and detection signals occurring for each focused state of the charged particle beam are accumulated with regards to signals detected by the detector. Each accumulated signal is then stored and an optimum focal point position is obtained from the stored series of accumulated values. The focusing lens is then set to the optimum focusing position. In this method, the focal point position is changed every time the vertical position of the scanning line changes rather than being changed once for every frame and image definition is compared for each scanning line. Control can therefore be implemented more rapidly compared with the related art where images are compared for every frame. However, in this method, there is a problem in that a pattern extending in a vertical direction of the sample image as shown in FIG. 1A is necessary in order to implement automatic focal point adjustment. If this is not present, it is not possible to perform a comparison of every scanning line. Namely, image definition for each scanning line can be discerned using the differences in image signals occurring at points passing through boundary regions. Therefore, when the image is a linear pattern going along the direction of the scanning lines as shown in FIG. 1B, the scanning lines do not pass through the boundary region and the focusing operation therefore does not operate with this method. In other words, the image information in the scanning direction in this case is uniform and difference signals for neighboring pixel information are therefore all zero. Semiconductor patterns differ from typical images taken of scenery or people in that vertical direction boundaries and horizontal direction boundaries are common, which means that such problems cannot be neglected in these kinds of situations.